This invention relates to electrical circuitry. More particularly, it is concerned with logic circuits which provide an interface between. two logic systems.
Two widely-used, well-known logic systems are ECL (emitter coupled logic) and CMOS (complementary metal oxide semiconductor) logic systems. The logic levels for ECL logic are -1.6 volts and -0.8 volt and for CMOS the logic levels are 0 volts and +5 volts. The threshold voltage for CMOS logic, that is the voltage at which a CMOS logic circuit triggers from one operating state to another, is +2.5 volts. Since this voltage is outside the operating range of ECL logic, CMOS logic circuitry is not directly compatible with ECL circuitry. CMOS logic circuits may be made compatible 25 with input from ECL circuitry by changing the operating voltages of the CMOS circuitry to -3.7 volts and +1.3 volts to provide a threshold voltage of -1.2 volts, the threshold voltage of ECL circuitry. Under operating conditions, however, variations in temperature or circuit parameters may cause changes in the current flow through the transistors thereby altering the threshold voltage of the circuit. Since the operating range of ECL logic is relatively narrow, even small changes can cause the circuit to become inoperable.